1. Technical Field
The present invention relates to semiconductor fabricating devices and more specifically to an ion implantation system for a semiconductor wafer.
2. Discussion of the Related Art
In general, electrical properties of a semiconductor crystal are modified by introducing controlled amounts of dopant impurities into the semiconductor crystal. Ion implantation and diffusion are the most commonly used methods for introducing impurities into the crystals of a semiconductor wafer. In a conventional semiconductor doping technique, positive-type (P-type) impurities such as boron or BF2, and negative-type (N-type) impurities such as arsenic, phosphorus and antimony are used as dopants.
Doping of semiconductor wafers by diffusion is done by introducing impurities into the semiconductor wafers and redistributing them within the semiconductor's crystals at an elevated temperature. Unlike diffusion, implanting ions is a low-temperature process in which ionized dopants are accelerated to high energies so that the dopants penetrate to a certain depth when they impact a target semiconductor wafer. Ion implantation has become one of the preferred methods for doping semiconductor wafers because of its flexibility in achieving different impurity profiles, and its ability to control the concentration of dopants.
FIG. 1 is a block diagram showing a conventional ion implantation system. The elements of the ion implantation system and/or machine (also known as an implanter) include an ion source 10, a beam transport 12, an end station 14 and a man-machine interface 16. The ion source 10 generates high-density ions, extracts a focused ion beam from the high-density ions and transfers an ion beam to a target wafer in the end station 14.
FIG. 2 is a schematic block diagram showing the beam transport 12 of FIG. 1. The beam transport 12 includes a mass analyzer 120, an accelerator 124, a focusing system 126, and a scan system 128. The mass analyzer 120 is used to select any one of several ion patterns from the ion source 10 using a strong magnetic field capable of separating ions according to a mass-to-charge ratio. After an ion beam leaves the mass analyzer 120, it is accelerated by the accelerator 124 to obtain a required kinetic energy. Next, the accelerated ion beam is focused by the focusing system 126. In order to sprinkle impurities conformally on a whole surface of a wafer 140 that is set up in the end station 14, the accelerated ion beam is absorbed horizontally or vertically by the scan system 128 located in front of the wafer 140. The scan system 128 generally includes X-scan plates 128a and Y-scan plates 128b. 
The man-machine interface 16 is used by an operator to control an accelerator voltage and ion implantation system parameters equal to an ion implantation quantity. The man-machine interface 16 is also capable of displaying different system parameters, such as a beam current, on a screen to enable an operator to continuously monitor the ion implantation process. The operator may also control the ion implantation process as needed. For example, the operator may control an X-plate voltage and a Y-plate voltage by controlling a control stick (or control switches).
A mode in which the ion implantation parameters are controlled by the operator is referred to as a beam setup mode. In the beam setup mode, if all system parameters are controlled, the ion implantation system is substantially converted to an ion implantation mode where the ion beam is scanned on the wafer 140. In the ion implantation mode, ion implantation quantity, beam current, implantation time etc., are automatically controlled by a conventional dose processor. The beam setup-to-ion implantation mode conversion is performed by an operator controlling a key switch at a remote console.
While parameters are manually controlled by the operator in the beam setup mode, they are automatically controlled by the dose processor in the ion implantation mode. During the beam setup mode, there is typically a request to prevent an ion beam from being transferred to the wafer 140 in the end station 14. Unlike the beam setup mode, during the ion implantation mode, an ion beam is transferred to the wafer 140 in the end station 14. This process is typically performed by a conventional Faraday assembly. For example, the Faraday assembly prevents an ion beam from being transferred into the end station 14 during the beam setup mode, and transfers an ion beam into the end station 14 during the ion implantation mode. Before the wafer 140 is loaded in a chamber of the end station 14 in the ion implantation mode, an ion beam is absorbed to prevent the loaded wafer 140 from being damaged. In spite of the ion implantation mode conversion, which takes place according to the control of the key switch, the ion beam remains in the chamber of the end station 14. This scenario will now be described with reference to FIG. 3.
Referring to FIG. 3, a target selection circuit comprises resistors R1 and R2, and a photo coupler 18. The photo coupler 18 includes a light-emitting diode LED 22, which is a light-emitting element, and a photo transistor 24, which is a receiving element. The LED 22 generates a light 26 because a constant voltage 15V is supplied through the resistor R1. The light 26 is transferred to the photo transistor 24. The photo transistor 24 then is converted and/or switched to a conductive-state after receiving the light. As a result, an output node 28 goes to a low level. The output node 28, when at the low level, represents the beam setup mode. A relay circuit 20 is operated when a key switch KEY_SW is pushed, and thus the photo coupler 18 stops operating. That is, the photo transistor 24 of the photo coupler 18 is converted to a non-conducting state. As a result, the output node 28 goes to a high level. The output node 28, when at the high level, represents the ion implantation mode.
When the key switch KEY_SW is switched to an on-state, the ion beam is moved from the chamber of the end station 14 to a dump region by controlling one of the scan plates 128a, 128b of the beam transport 12. As a control path of the beam transport 12 is converted from the remote console to the dose processor, the ion beam is automatically moved to the dump region by the dose processor. If the constant voltage of 15V is continuously applied by inputting it to the photo coupler 18 because of inadequate contact being made by the relay circuit 20, the output node 28 is continuously at the low level thus representing the beam setup mode. In other words, the ion beam is continuously transferred to the wafer 140 when the wafer 140 is loaded in the chamber of the end station 14. As a result, there is a wafer loss. Therefore, there is a need for preventing an unnecessary ion implantation process by detecting inadequate contact of the relay circuit 20.